Citrix open sources XenServer hypervisor to chase clouds

The XenServer hypervisor is going back to its open source roots, now that Citrix Systems has decided to let go of the code behind its commercial-grade variant of the Xen server virtualization tool.

Xen came out of Cambridge University by storm a decade ago, spawning XenSource to offer tech support for the product. For a while in the mid-2000s, XenSource was the only real rival to VMware for x86 server virtualization, and that's why Citrixshelled out $500m in August 2007 to acquire the company, which had something on the order of $1m in sales at the time.

One could argue that the infusion of XenSource techies and the Xen hypervisor and related tools saved Citrix from oblivion, and in that regard the Xen acquisition more than paid for itself. But XenServer itself has not been a great moneymaker for Citrix, even if it is a foundational technology.

Xen is popular on public clouds, and the company wants to expand the use of Xen and its commercial-grade variant, XenServer, on public and private clouds.

Amazon Web Services chose Xen as its virtualization technology to build its EC2 compute cloud eight years ago, and Rackspace Hosting uses the commercial XenServer variant to virtualize the servers underneath its public cloud, just to name the two most important Xen users.

Moreover, XenServer is the hypervisor of choice to dice and slice servers to run virtual desktop infrastructure (VDI) – cloudy PCs, really – from the data center when companies choose the XenDesktop VDI broker, which in fact does make Citrix lots of money.

And Citrix needs to keep it that way.

In each of those cases, Citrix wants to make some money, but perhaps more importantly, it needs to keep other hypervisor camels from getting their noses under the data center tents.

And so the company is opening up the full-on XenServer hypervisor, including lots of bits that were proprietary extensions, as well as the XenCenter console for babysitting hypervisors and virtual machines that is its constant companion.

With this move, XenServer can now be embraced by public and private cloud builders who want something that is free – or at least inexpensive – but yet sophisticated enough to do the kinds of things clouds need.

It also means that XenServer can be paired more naturally and cleanly with CloudStack, the cloud control freak thatCitrix bought for an estimated $500m back in July 2011 and fully open sourced at the Apache Foundationin April 2012. Finally, it also means that XenServer can be a peer to KVM on the rival OpenStack cloud controller.

By the way, the open source Xen hypervisor project that was absorbed into the Linux Foundationback in April to give Amazon, Google, and others more sway into its development, continues on as a separate open source project. XenServer will take the core Xen code and wrap high availability and other tools such as XenCenter around the core Xen hypervisor and do so in such a way that companies can use it in production and get support contracts from Citrix to make them feel safe.

XenServer and XenCenter are being open sourced under a mix of GPL, BSD, and Citrix licenses, depending on the component. You can see the full component list and their licenses here.

What Citrix is not trying to do, explains Krishna Subramanian, vice president of product marketing for the cloud division, is to take on Microsoft's Hyper-V and ESXi hypervisors for all workloads. Rather, it is positioning XenServer as a value leader for clouds and VDI, with a special emphasis on clouds.

"Clouds tend to be greenfield installations, and their builders are very price-sensitive on the hypervisor front," Subramanian tells El Reg. "We have had over one million downloads of XenServer, and we are getting hundreds of thousands of downloads per quarter for it now. These companies want a hypervisor layer that just works and doesn't cost much. While some companies will use VMware for certain workloads, the license fees for VMware really add up when you are building a cloud."

And thus, Citrix is doing away with the XenServer Basic, Enterprise, and Platinum Editions and their tiered feature sets and pricing, and is offering a single XenServer with a simple price: if you want a perpetual license to support for XenServer 6.2, which was just announced, you pay $1,250 per socket (that includes a 25 per cent maintenance fee), and if you want to go with a subscription, then you pay $500 per socket per year.

That's it.

And while it may not be as cheap as Hyper-V (which is free), that makes it considerably cheaper than VMware's ESXi hypervisor and competitive with Red Hat's KVM server slicer.

The opening up of XenServer comes concurrent with the 6.2 release of the hypervisor, which starts out by supporting the "Piledriver" Opteron 3300, 4300, and 6300 processors announced by AMD last November, as well as the new "Haswell-DT" Xeon E3 and impending "Ivy Bridge-EP" Xeon E5 and "Ivy Bridge-EX" Xeon E7 processors from Intel. The latter are due in the third and fourth quarters, respectively.

The updated XenServer 6.2 supports up to 500 virtual machines per host, and can have as many as 3,250 virtual CPUs on that host system; it is based on the core Xen 4.1.5 hypervisor. XenServer 5.5/5.6 could do 50 virtual machines per host and XenServer 6.1 could handle 150 virtual machines; these are tested limits on then-current systems, not theoretical limits. It is not clear as we go to press how many vCPUs the earlier XenServer releases could support, or how much virtual memory. Subramanian says that on performance tests, XenServer 6.2 can have as much as 40 per cent better performance (meaning placing less overhead on VMs) than the 6.1 release. It is also not clear what the tested virtual memory and virtual CPU limits are for individual VMs running atop XenServer 6.2, and it is not in the release notes, as far as we can tell. Ditto for the memory and I/O scalability of the hypervisor itself. (See Bootnote below for an update.)

With the 6.2 release, Microsoft's Windows 8 and Windows Server 2012 move from experimental to official support within the XenServer virtual machines, and there are tweaks to make support for Windows Server 2008 R2 better, specifically relating to the Volume Shadow Copy Service, or VSS – and yes, that is Microsoft's abbreviation.

Commercial-grade Linuxes from Red Hat, SUSE Linux, Oracle, CentOS, and Debian were also enhanced with dot release updates and other tweaks with XenServer 6.2.

Not everything made the jump from proprietary XenServer to the open source XenServer with the 6.2 release. The Integrated StorageLink software, which allowed XenServer to access the snapshotting, replication, and thin provisioning features of storage arrays, is deprecated, as is the Distributed Virtual Switch Controller and the hooks into Microsoft's Systems Center Virtual Machine Manager. These tools work as-is at the 6.1 level, with the 6.2 release, but they are not going to be enhanced. Open vSwitch, which is controlled by VMware and as the name suggests is open source, is still part of the XenServer stack.

One other important thing: the XenCenter console for XenServer used to have tiered functioning, just like XenServer itself, and all of that is gone, now that XenCenter is opened up like XenServer itself is – with one exception. All the features and functions of XenCenter are available to customers using the open source code or freebie binaries except for the ability to use XenCenter to apply security and feature hotfixes. You can still apply fixes by hand to XenServer if you want to, but to use XenCenter, you will need a support contract. ®

*Bootnote: XenServer engineer Thomas Sanders, who is wiser to the ways of the Citrix online documentation than El Reg, points out that in addition to release notes for XenServer, Citrix also publishes a Configuration Limits guide as well as a Technical FAQ for each XenServer release. (Those links take you to the limits for XenServer 6.2.) With the 6.2 release, the VM limits are exactly the same. You can have 16 virtual CPUs, 128GB of virtual memory, and just a hair under 2TB of virtual disk affiliated with a XenServer virtual machine. On the hypervisor itself, the logical processors per host is topped out at 160 on the current and prior release, but with XenServer 6.2, the number of virtual CPUs per host is now capped at a theoretical 4,000 (with 3,250 tested according to the release notes) compared to 900 for XenServer 6.1. That is for a Windows machine. If you have a Linux machine, you can scale to 12,000 virtual CPUs, in theory, if you can get someone to build you such a massive box. The number of concurrent VMs that can be running per host has also jumped, from 150 with XenServer 6.1 (for both Windows and Linux guests) to 500 for Windows VMs and 650 for Linux VMs. The main memory for the hypervisor still tops out at 1TB, which is looking a bit skinny but is still alright.